Loudspeakers



Last edited: March. 31, 2020

Explanations of the 'boxy sound', extraordinary speaker designs, full-range drivers, two-way vs. three-way speakers, the missing specs, speaker spikes or rubber feet, why desktop speakers sound bad

The most important component of an audio system is the loudspeaker and the headphone. Modern audio players and amplifiers are so perfect that it makes no sense to talk about audible differences anymore. On the other hand the difference between loudspeakers and headphones is still huge. Most loudspeakers have a nonlinear frequency response (may come from driver resonances, uncompensated baffle diffraction, excessive cabinet edge diffraction, etc.) or other problems (poor off-axis responses). In addition to that the speaker's frequency response will vary according to room size, speaker placement, listening distance... Reflection from the wall behind the speaker and from the floor (or from the table for desktop speakers) is a common problem in room acoustics.


What makes a loudspeaker's sound 'boxy'?

Explanations of the 'boxy sound' - what makes a loudspeaker's sound 'boxy':

Some notes on baffle step compensation (BSC). Although baffle step compensation is the popular term, it is not accurate. The correct term is free-field equalized speaker which means that the speaker's frequency response set to flat in free-field or quasi free-field (outdoors is close spproximation of free-field). A free-field equalized speaker may have too much bass if set up close to the rear wall - especially at high volume. Baffle step compensation is common in high-end loudspeakers and in loudspeaker kits. Mini HiFi speakers, low-cost floor-standing and bookshelf speakers have no compensation and active monitors have adjustable low frequency tuning. Of course, there are exceptions to this rule. Other names for baffle step: baffle loss, diffraction low frequency loss, low frequency loss in full space.


Extraordinary speaker designs

Most extraordinary speaker designs have no any sonic advantage over the traditional two-way / three-way speaker design:


Which way?

Single full-range drivers (aka one-way speakers) are a compromise compared to multi-way speakers. The problem with full-range speakers is that no driver can reproduce the full audio range. Reproduction of bass frequencies requires large cone diameter with large cone excursion. Reproduction of the treble range needs small cone, and the cone excursion is less important. Small full-range speakers in the 2-3 inch diameter range have good high frequency response, but limited bass, and on the contrary large full-range speakers have more bass but they lose in the treble. Three inch full-range drivers are a good balance between size and frequency response in small desktop speakers.

Two way vs. three way speakers. None of the two designs have any magical qualities compared to the other. Three way design make sense only if the woofer is larger than 6 inches. Three way configuration only simplifies tweeter selection and crossover design.


The missing specs

Sadly loudspeaker 'spec sheets' are not very informative and the frequency response measurement is always missing. Some typical spec sheet data: nominal impedance, voltage sensitivity in dB (SPL from 1m with 2,83V at the terminals, full space), 'recommended amplifier power' and the speaker's dimensions. Fortunately, some websites publish loudspeaker reviews with objective measurements:

It's possible to correct the uneven frequency response of any loudspeaker with an equalizer. Frequency response errors that can be fixed with an EQ: poor low frequency tuning (resonance), baffle step issues, cone-surround resonance (the first bump in the frequency response of cone speakers), level differences, attenuation and boosting of frequency ranges. Other speaker flaws cannot be corrected this way, such as internal standing wave resonances, alu cone resonances, large directivity issues, too low Xmax (too low SPL in the bass) etc... Equalization (frequency response correction with a microphone measurement) can turn an unlistenable bad speaker into an audiophile speaker.


Speaker spikes

The design and the material of speaker spikes or speaker feet is an eternal question; another misunderstood topic where customers and the audio industry goes into crazy levels (as with audiophile cables). Obviously, cars are fitted with tires just to keep the roads from wearing out so fast... Well, not really... Stone or metal have much worse damping and isolation performance than rubber, so rubber feet are not only cheaper, but also better.

There is a simple test for this. Put a smaller bookshelf or desktop sized speaker with good bass on a large and strong table, and put different materials under the speaker like felt, rubber or screw nut. Turn on some music and put your hands on the table. Rubber feet lowers the vibration better than metal spike, although spike is better than nothing. (And be careful not to scratch the table with metals.)


Case study: why small desktop speakers sound bad?

Small desktop speakers are not associated with high fidelity. Usually a three inch full-range driver built in an undersized vented box that pushes the resonance frequency even higher. These speakers have a peak about 200 Hz and this peak is responsible for the muddy bass.

Logitech X-120 frequency response measurement

This graph shows the free field frequency response of the Logitech X-120 (speaker from ~2010). This is a decent speaker in its category, quite loud, but the amp section is a bit noisy. The peak between 200 and 300 Hz is quite annoying and removing only this peak can give a new life to this speaker. (The small rise between 500Hz and 1500 Hz is the baffle step, due to the high resonance the baffle step is 'lowered'.)

Csaba Horvath

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